A momentary electrical event occurring through the membrane of a nerve cell fiber in response to a stimulus, forming a nerve impulse.
An action potential is transmitted along a nerve fiber as a wave of changing electrical charge. This wave travels at a speed that ranges from about five feet (1.5 m) per second to about 350 feet (107 m) per second, depending on various properties of the nerve fiber involved and other factors.
An action potential occurs in about one millisecond. During an action potential, there is a change in voltage across the nerve cell membrane of about 120 millivolts, and the negative electrical charge inside the resting nerve cell is reversed to a positive electrical charge. This change in voltage and reversal of electrical charge results from the movement of sodium ions, which carry a positive charge, into the nerve cell fiber. This is followed by the movement of potassium ions, which also carry a positive charge, out of the nerve cell fiber, allowing the nerve cell to return to its resting state. The temporarily increased permeability of the nerve cell fiber membrane, first to sodium ions and then to potassium ions, is caused by a chemical transmitter substance.
Adams, Raymond. Principles of Neurology. New York: McGraw-Hill, 1993.